In automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, mass flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which as sensors register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity value, respectively.
Serving for influencing process variables are so called actuators, e.g. valves, which control the flow of a medium in a pipeline section.
A large number of such field devices are produced and sold by the firm, Endress+Hauser.
Frequently, field devices are connected with superordinated units via a 4-20 mA electrical current loop. These superordinated units can serve for process control, process visualizing and process monitoring. A large number of the field devices used in the present state of the art are two-wire-, or four-wire-devices. Two conductor devices are connected via a line-pair with a receiver unit, which serves as superordinated unit. These devices are supplied with electrical current exclusively via the signal line. The electrical current, which flows via the signal line, serves for signal transmission between the field device and the receiver unit. An industrial standard for this type of signal transmission is the 4-20 mA technology.
In the case of two conductor devices, the provided input power is significantly limited. The electronics in the field device must be so embodied that it works reliably even in the case of a minimum signal current of 4 mA. As a rule, a microprocessor is present in the electronics and serves for measured value processing. The low provided input power represents a limiting factor especially also in the case of the visual status display. While the status display should be as bright as possible, there is, however, only a limited input power available.